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Heat pumps · Error codes · NIBE
334NIBE · NIBE, all models

334 on a NIBE heat pump: Max. incoming temp. exceeded: Max temperature through outdoor unit exceeded: BT3 higher than 65 C in heating mode

Short answer

On a NIBE heat pump, 334 means: Max. incoming temp. exceeded: Max temperature through outdoor unit exceeded: BT3 higher than 65 C in heating mode. The heat pump keeps running with some functions stopped.

Reviewed 23 September 2026 · 2 sources

What it means

The manuals for this range word it in different ways:

  • Max. incoming temp. exceeded: Max temperature through outdoor unit exceeded: BT3 higher than 65 C in heating mode.
  • Failed compressor start: Compressor does not start when required, 3 times within 2 hours [other NIBE documents: Compressor temporarily ran below minimum permitted speed 3 times within 2 hours or for 1 hour].

Part involved: heating medium flow, charge pump, particle filter; inverter and compressor drive.

When it shows: Max temperature through outdoor unit exceeded: BT3 higher than 65 C in heating mode Compressor does not start when required, 3 times within 2 hours; Compressor temporarily ran below minimum permitted speed 3 times within 2 hours or for 1 hour.

Likely causes:

  • Undersized heating system; incorrect heating curve; flow-related; additional heat incorrectly connected.
  • S2125: Defective inverter; defective compressor | F2120: Defective inverter; defective compressor.

Is it safe?

Does the heat pump stop?Partly
SeverityWarning

The heat pump keeps running with some functions stopped.

Resetting itself: yes - Heating prioritisation blocked.

Manual reset: Reset from the controller alarm menu ('reset alarm' / 'Reset alarm and try again'); alarm recurs if the cause remains.

What you can check

  • Read the alarm text and the 'info/action' help on the controller display (VVM/SMO or S-series smartguide). NIBE's manuals let the homeowner select 'reset alarm' ('Reset alarm and try again' on S-series); if the lamp returns to normal the alarm has cleared. If it comes back, do not keep resetting: select 'aid mode' ('Auxiliary operation' on S-series) if offered, so any electric additional heat keeps heat/hot water going, and contact your installer. Read the heating system pressure gauge and check that radiator/underfloor thermostats are not all closed; check heating and hot water temperature settings have not been changed.
  • Read the alarm text and the 'info/action' help on the controller display (VVM/SMO or S-series smartguide). NIBE's manuals let the homeowner select 'reset alarm' ('Reset alarm and try again' on S-series); if the lamp returns to normal the alarm has cleared. If it comes back, do not keep resetting: select 'aid mode' ('Auxiliary operation' on S-series) if offered, so any electric additional heat keeps heat/hot water going, and contact your installer. Check the heat pump and indoor unit breakers/RCD at the consumer unit are on; if one trips again, leave it off and call an engineer.

What the engineer will check

  • Check heating medium flow (pump speed/settings, particle filter, air in system), heating curve and hot water settings, correct sensor placement and flow/return connections. NIBE lists: Undersized heating system; incorrect heating curve; flow-related; additional heat incorrectly connected.
  • Check main and group fuses and cable connections, supply voltage under load, inverter terminal blocks, inverter mounting/cooling paste, compressor wiring and compressor. NIBE lists: Defective inverter; defective compressor.

Who to ask for: NIBE-trained heat pump installer (NIBE PRO / VIP); NIBE-trained heat pump engineer and/or qualified electrician.

Certification: None specific beyond competent heat pump installer; electrical work to BS 7671; Electrical work to BS 7671 by a competent electrician (notifiable work under Part P in England & Wales).

What you might notice

Sources

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026